Numerical Experiments on Bore Merging over Mildly Sloping Beaches

The phenomenon of bore merging, in which faster bores overtake slower ones and end up merging, is commonly observed in natural surf zones and can significantly modify the wave field. Bore merging is associated to a decrease in the number of waves and thus to an increase of the mean wave period. As such, it can be seen as an energy transfer mechanism to lower frequencies. This led several authors to hypothesize that this process could contribute to infragravity wave growth, although it is unclear if this contribution is significant. Recent studies additionally suggest that bore merging is a driver for extreme run-up and could contribute to sediment transport. However, despite its potentially high relevance to nearshore dynamics, few studies have analyzed the processes leading to bore merging and its effect on both the short- and infragravity-wave field in the surf zone. Here, we use a series of numerical experiments to gain new insights into the mechanisms driving bore merging over mildly sloping beaches and to characterize its effect on the wave field. More specifically, we use a numerical model based on nonlinear shallow water equations to simulate wave transformation in the inner surf zone for boundary conditions varying in terms of short and infragravity wave characteristics. This allows us to analyze how bore merging is influenced by, but also influences, the infragravity wave field. We also use our numerical experiments to investigate the relative importance of amplitude dispersion in the process, which has been long considered as the primary driver for bore merging. Finally, we examine how the bore merging influences the overall energy distribution in the surf zone, including its effect on breaking-induced energy dissipation.